Neublastin, also known as artemin and enovin, is a 24 kDa homodimeric, secreted protein that promotes the outgrowth and survival of neurons of the peripheral and central nervous system (Baudet et al., 2000, Development, 127:4335; Masure et al., 1999, Eur. J. Biochem., 266:892; Rosenblad et al., 2000, Mol. Cell. Neurosci., 15(2):199). Neublastin mRNA is expressed predominantly in embryonic kidney and lung, and in adults, is expressed highest in pituitary gland, trachea, and placenta (Baudet et al., 2000, Development, 127:4335).
Neublastin is a member of the glial cell line-derived neurotrophic factor (GDNF) ligand family. GDNF ligands activate both Ras and phosphatidylinositol-3-kinase signal transduction pathways by engaging the membrane-bound c-RET receptor tyrosine kinase. This c-RET-mediated signaling requires an additional co-receptor, a glycosylphosphatidyl inositol (GPI)-anchored GDNF family receptor alpha (GFRα) protein, which confers ligand specificity to c-RET. Four GFRα co-receptor proteins have been identified (GFRα1-4). Neublastin shows highest affinity for GFRα3 in vitro, however in studies using human fibroblasts, neublastin can stimulate c-RET-dependent signaling through either GFRα3 or GFRα1 (Baudet et al., 2000, Development, 127:4335; Masure et al., 1999, Eur. J. Biochem. 266:892; Rosenblad et al., 2000, Mol. Cell. Neurosci. 15(2):199).
Neublastin and the other GDNF family members are members of the transforming growth factor beta (TGF beta) superfamily and thus, are characterized by the presence of seven conserved cysteine residues with similar spacing which form the structure of a cysteine knot (Saarma, 1999, Microsc. Res. Tech., 45:292). Each monomer contains two disulfide bonds that form a closed loop structure encircling the third disulfide to form a tight knot structure. The seventh cysteine contained within each monomer forms an intermolecular disulfide bond, covalently linking the monomers to form the final dimer product (Rattenholl et al 2000, J. Mol. Biol., 305:523).